Most, if not all, musical instruments produce varying degrees of harmonic change with time as they are played. Some instruments exhibit this change during the onset of tone production, and then settle down harmonically to a "steady state" condition. Examples of such instruments would be horns, bowed strings, and organ pipes. There are other instruments where the harmonic change occurs throughout the audible sound production. Examples of these would be plucked strings, bells, and piano-type instruments. In synthesizing the sounds of musical instruments electronically, harmonic changes with time are an important contribution to realism.
In presently manufactured electronic musical instruments, harmonic variations with time have been implemented with varying degrees of success. One such approach is described in U.S. Pat. No. 4,184,403, assigned to the assignee of the present invention wherein waveform generation is accomplished by successively reading out amplitude samples of a waveform from a memory. The harmonic content of the voice of audible tone being generated is caused to change by reading from multiple memories, singly but in sequence, where each memory contains a slightly different harmonic content. The major deficiency in producing audible tones in the manner just described is the fact that under certain circumstances, such as a long, gradual decay, it becomes apparent to the listener that a sequence of discrete harmonic structures is being generated as opposed to a smooth gradually changing harmonic sequence. In a co-pending application entitled Transient Harmonic Interpolation for an Electronic Musical Instrument, invented by J. T. Whitefield and R. P. Woron, Ser. No. 272,223, filed June 10, 1981, and now U.S. Pat. No. 4,352,312, the major deficiency explained above was eliminated. However, this approach, along with the approach of U.S. Pat. No. 4,184,403, are such that the frequency of harmonic change or, in other words, the harmonic change per unit time, is uniform throughout the entire harmonic sequence and thus are subject to the size of the harmonic waveform memory. That is to say that twice as much change per unit time implies a need for twice as much memory space.
It is therefore an object of the present invention to eliminate the constraints of uniformity of the sequence of discrete harmonic structures by causing the transient harmonic interpolation to occur over a period of time less than the total transient time.
It is another object of the present invention to be able to cause the transient harmonic interpolation to occur over the entire transient period but in a non-linear fashion so that the degree of interpolation at different times during, for example, percussive decay, can be specifically tailored for that period of the tone.
Other objects will appear hereinafter.